Abstract: A gravity-powered zip line trolley return system for a zip line system having a zip line cable and a trolley configured to traverse the zip line cable may include a mounting block detachably mountable to the zip line cable of the zip line system. A spool bracket may be supported by the mounting block. A first spool may be supported by the spool bracket. A second spool supported by the spool bracket, wherein the first spool and the second spool are rotationally coupled to each other through a gear reduction ratio mechanism and a drive belt. A trolley return line may be windable on the first spool and connectable to the trolley of the zip line system. The trolley return line may be configured to move with the trolley in a forward movement and to pull the trolley in a backward movement as the first spool rotates. A counterweight suspension element may be windable on the second spool. A counterweight may be attached to the counterweight suspension element.